Energy Variables Or Energize Me Mechanical Energy Mechanical

Energy Variables Or Energize Me!

Mechanical Energy • Mechanical Energy is the ability to cause changes in – Speed – Elevation – Compression of a spring – Motion of particles inside object • Measured in Joules (kg m 2 /s 2 ) =energy to raise a quarter pounder 1 m in 1 second • Scalar Quantity • Usually measure changes in energy • Conserved quantity when have no outside forces except gravity and springs

Kinetic Energy • Things have more energy when – They move faster – They are bigger move at the same speed as small things • Kinetic energy is the energy due to an object’s motion • The clay experiment • DKE = ½ mvf 2 - ½ mvo 2 – Positive change means object is speeding up

Gravitational Potential Energy • Things have more energy – when they have a higher elevation relative to some reference level – if they have more mass at the same elevation as less massive things • Gravitational Potential Energy is the energy an object stores due to its position in a gravitational field • DPE gravity = mg (Dh) • You decide the reference level • Only vertical displacements change PE g

Elastic Potential Energy • Springs have more energy – that are stretched or compressed more – that are stiffer given the same stretch • Elastic potential energy is the energy stored in a spring or other elastic object. • DPE elastic =1/2 k Dxf 2 - 1/2 k Dx o 2

Work • More energy is transferred by – Large magnitude forces than small forces – Forces that act on objects for a larger distance – A force exerted perpendicular to the motion of an object does not affect the object’s speed • Work is the transfer of energy due to a force acting in the same or opposite direction as the motion of an object. • Scalar quantity measured in joules • W= F x d cos q where theta is the angle between the force and distance moved. • Positive work increases overall energy, negative work decreases overall energy.

The Work-Energy Theorem • W = Fd cos q = DKE + DPE gravity + DPE elastic – Work is like the ATM – KE, PE gravity and PE elastic are the accounts – True as long as work changes only these accounts • W = Fd cos q = DKE = ½ mvf 2 - ½ mvo 2 – True as long as work only changes the KE of an object i. e. no elevation changes or spring changes – True as long as mass of object remains constant – Work-kinetic energy theorem

Power • Power is the rate at which work is done • Power is measured in watts or horsepower – 1 watt = 1 joule /second – 750 joules = 1 horsepower • Power is a scalar • Power = work in joules /time in seconds • Powerful machines do more work in the same time or the same work in less time compared to less powerful machines.